GB618787A - Production of optical elements - Google Patents

Abstract

Optical elements which possess an axis of reference such that the thickness of the element <PICT:0618787/IV(a)/1> <PICT:0618787/IV(a)/2> measured parallel to the said axis at all points in the element at a given distance from the said axis is substantially constant are prepared by subjecting a liquid polymerizable monomeric compound having the group >C=C<, or a liquid partial polymer derived therefrom, to the polymerizing action of light in a transparent mould giving the shape of the said optical element until a solid body is formed, the average quantity of light falling per unit time upon the said monomer or partial polymer being controlled according to the equation Qx=ktx (1-Kx) wherein Qx is the average quantity of light falling per unit time upon the polymerizable compound at a distance x in the centre of the optical element from the said axis of reference, tx is the thickness of the material being polymerized, measured in the direction of the incident light, at the same distance x, k is an arbitrary constant and K has any positive value greater than O and less than 1/r, where r is the maximum value of x, that is, the semi-diameter of the optical element. If the light is not parallel to or symmetrical about the axis of reference there will be several values of tx for each value of x and then an average tx is taken. Usually K is about 1/10 r but is chosen by trial and error to be the value giving a bubble free casting, while Wx is related to a small unit of time such as a minute or second. Monomers specified are acrylic and methacrylic acids, methyl methacrylate, cyclohexyl methacrylate, 2,2,2,-trifluoroethyl methacrylate, fluoroacrylic esters with fluorinated alcohols, styrene, vinyl acetate, allyl methacrylate, diallyl fumarate. Low molecular weight polymethyl methacrylate, obtained by malaxating higher polymers on hot rolls, or by polymerizing the monomer in the presence of a large amount of catalyst or in the presence of a chain ending compound such as turpentine, as well as mixtures of monomer and polymer may be used. Photopolymerization catalysts such as glycollic aldehyde; benzoin; acetoin; butyroin; 12 - hydroxy - 13 - ketotetracosane; o, and p tert. butyltoluin, benzoin methyl ether; benzoin ethyl ether; benzoin propyl ether; pivaloin ethyl ether; anisoin methyl ether; diacetyl; pentadione-2,3; octadione-2,3; 1 - phenyl - butadione - 1,2; benzil; 2,2-dimethyl-4-phenylbutanedione-3,4; glyoxal; phenyl-glyoxal; diphenyl triketone; and 1,2-cyclohexanedione may be used with or without an organic peroxide such as benzoyl, lauroyl, acetyl, and butyroyl peroxides or heat and light active catalysts such as a ,a 1-azodiisobutyronitrile; a ,a 1-azo bis-(a ,g -dimethylvaleronitrile); dimethyl and diethyl a ,a 1-azodiisobutyrates; 1,11-azodicyclohexanocarbonitrile; a ,a 1-azobis-(a -ethylbutyronitrile); a ,a 1-azobis-(a -ethylbutyronitrile); and a ,a 1-azodiisobutyrocarbonamide. Control of Qx may be effected by having an excised or transparent sector in an opaque screen which is interposed between the source of light and the mould, the sector covering all parts at which, using polar co-ordinates, <FORM:0618787/IV(a)/1> , taking the centre of the screen as origin and any radius of the disc as the x axis. The screen may have more than one sector of the required shape, there being relative rotation between the screen and mould. A photographic plate may be placed behind the screen so that on exposure with relative rotation of the screen and plate a negative is obtained, the positives of which having a circular area allowing the controlled amount of light Qx to pass, relative rotation between the mould and such a positive being unnecessary. Allowance is made in the sector for the forming of a flash for the optical element. In Figs. 1 and 2 the parabolic reflector 1 containing a lamp is mounted over an opaque screen 2 having an accurately cut sector 3. Below the screen is a mould made of two halves 4 and 5 separated by a gasket 8 and mounted on a turntable 9 rotatable by the spindle 10. The portion 13 of the sector 3 allows for polymerization of the flash. A graded diffusion screen may be interposed between the lamp and the screen to give a uniformly bright source.